Method of fabricating crystalline island on substrate

ABSTRACT

Certain electronic applications, such as OLED display back panels, require small islands of high-quality semiconductor material distributed over a large area. This area can exceed the areas of crystalline semiconductor wafers that can be fabricated using the traditional boule-based techniques. This specification provides a method of fabricating a crystalline island of an island material, the method comprising depositing particles of the island material abutting a substrate, heating the substrate and the particles of the island material to melt and fuse the particles to form a molten globule, and cooling the substrate and the molten globule to crystallize the molten globule, thereby securing the crystalline island of the island material to the substrate. The method can also be used to fabricate arrays of crystalline islands, distributed over a large area, potentially exceeding the areas of crystalline semiconductor wafers that can be fabricated using boule-based techniques.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/184,429, filed on Jun. 16, 2016 and published as US2016/0300716, which is incorporated herein by reference in its entirety.U.S. patent application Ser. No. 15/184,429, in turn, is acontinuation-in-part of U.S. patent application Ser. No. 14/610,567,filed on Jan. 30, 2015 and published as US 2015/0357192, which is alsoincorporated herein by reference in its entirety. U.S. Ser. No.14/610,567, in turn, claims priority from U.S. Provisional PatentApplication No. 62/007,624, filed on Jun. 4, 2014, which is alsoincorporated herein by reference it its entirety.

FIELD

The present specification relates to a method of fabricating one or morecrystalline islands of an island material abutting a substrate.

BACKGROUND

Certain electronic applications, such as OLED display back panels,require small islands of high-quality semiconductor material distributedover a large area. This area can be 50 inches diagonally and larger, andexceeds the sizes of crystalline semiconductor wafers that can befabricated using the traditional boule-based techniques.

WO 2013/053052 A1, incorporated herein by reference, disclosesfabricating a large number of small, loose, crystalline semiconductorspheres. The spheres are then distributed on a patterned substrate andaffixed to the substrate at predetermined locations to form an array ofspheres on the substrate. Planarizing the spheres exposes across-section of each sphere, thereby providing an array ofhigh-quality, crystalline semiconductor islands for device fabricationon a globally planarized surface.

U.S. Pat. No. 4,637,855, incorporated herein by reference, disclosesfabricating spheres of silicon on a substrate by applying a slurry ofmetallurgical grade silicon to the substrate and then patterning theslurry layer to provide regions of metallurgical silicon of uniformsize. The substrate is then heated to melt the silicon, which then beadsto the surface to form molten spheres of silicon, which are then cooledto crystallize them. The spheres have very weak adhesion to thesubstrate and are easily detached from the substrate by simply knockingthem loose. The loose spheres are collected and undergo furtherprocessing.

US 2012/0067273 A1, incorporated herein by reference, disclosesfabricating silicon wafers by bringing a substrate into contact with areservoir of molten silicon, forming a layer of solid silicon on thesubstrate, and subsequently detaching the solid layer from thesubstrate. The disclosed method can potentially be used to fabricatesilicon wafers with large areas.

SUMMARY

Provided herein is a method of fabricating one or more crystallineislands of an island material abutting a substrate. For each crystallineisland, particles of the island material are deposited abutting thesubstrate, and the substrate and the particles are then heated to meltand fuse the particles to form a respective molten globule. Thesubstrate and the respective molten globules are then cooled tocrystallize the molten globules, thereby securing the crystallineislands to the substrate.

This method can allow for fabricating crystalline islands usingparticulate starting materials. In addition, in some implementations thecrystalline islands being secured to the substrate can allow for furtherprocessing of the islands; for example, planarizing at least a portionof each crystalline island to expose a cross-section of each island. Ifthe crystalline islands are sufficiently high-quality crystallinesemiconductors, these cross-sections can then be used to fabricateelectronic devices. This method can also allow for fabricating arrays ofcrystalline islands, distributed over an area potentially exceeding theareas of crystalline semiconductor wafers that can be fabricated usingtraditional boule-based techniques.

According to an aspect of the present specification, there is provided amethod of fabricating a crystalline island of an island material, themethod comprising depositing particles of the island material abutting asubstrate, heating the substrate and the particles of the islandmaterial to melt and fuse the particles to form a molten globule, andcooling the substrate and the molten globule to crystallize the moltenglobule, thereby securing the crystalline island of the island materialto the substrate.

The crystalline island can comprise a single crystal of the islandmaterial or a polycrystalline form of the island material.

The method can further comprise planarizing at least a portion of thecrystalline island to expose a cross-section of the crystalline island.

The securing can comprise the molten globule wetting the substrate at awetting angle smaller than about 90 degrees and the crystalline islandadhering to the substrate.

The depositing can comprise defining a depression in the substrate, andtransferring the particles of the island material into the depression.

When the depositing comprises defining a depression and transferring theparticles of the island material into the depression, the securing cancomprise a portion of a surface of the depression enveloping a portionof a surface of the crystalline island.

When the depositing comprises defining a depression and transferring theparticles of the island material into the depression, the depression canbe shaped to have at least one vertex.

When the depositing comprises defining a depression and transferring theparticles of the island material into the depression, the depression cancomprise a first depression, and a second depression within the firstdepression, the second depression smaller and deeper than the firstdepression.

When the depositing comprises defining a depression and transferring theparticles of the island material into the depression, the transferringcan comprise one or more of doctor-blading the particles of the islandmaterial into the depression, and electrostatic deposition of theparticles of the island material into the depression using a chargedpin.

When the depositing comprises defining a depression and transferring theparticles of the island material into the depression, the transferringcan comprise flowing a suspension onto the substrate and into thedepression, the suspension comprising a dispersion of the particles ofthe island material in a carrier medium, and squeegeeing the suspensionlocated on the substrate outside the depression; and the heating canfurther comprise eliminating the carrier medium prior to the melting andfusing the particles of the island material.

The cooling can comprise one or more of oxidizing an outer surface ofthe molten globule, super-cooling the molten globule, and applying aphysical impact to the substrate.

The depositing can comprise transferring particles of the islandmaterial into a through hole in the substrate.

When the depositing comprises transferring particles of the islandmaterial into a through hole in the substrate, pressure can be appliedat a second end of the through hole to push the molten globule partiallyout of a first end of the through hole to form a convex meniscus.

When the depositing comprises transferring particles of the islandmaterial into a through hole in the substrate, the securing can comprisea portion of a surface of the through hole enveloping a portion of asurface of the crystalline island.

When the depositing comprises transferring particles of the islandmaterial into a through hole in the substrate, the method can furthercomprise after the cooling to crystallize the molten globule,planarizing a portion of the meniscus to expose a cross-section of thecrystalline island.

When the depositing comprises transferring particles of the islandmaterial into a through hole in the substrate, a portion of a surface ofthe substrate outside the through hole and adjacent a first end of thethrough hole can have a wetting angle with the molten globule of lessthan about 90 degrees.

The depositing can comprise dispersing the particles of the islandmaterial in a carrier medium to create a suspension and transferring thesuspension onto the substrate, and the heating can further compriseeliminating the carrier medium prior to the melting and fusing theparticles of the island material.

When the depositing can comprise dispersing the particles of the islandmaterial in a carrier medium to create a suspension and transferring thesuspension onto the substrate, and the heating can further compriseeliminating the carrier medium prior to the melting and fusing theparticles of the island material, the transferring can comprise one ormore of stamping the suspension onto the substrate, screen printing thesuspension onto the substrate, inkjet printing the suspension onto thesubstrate, and spin-coating the suspension onto the substrate, andlithographically patterning the spin-coated suspension.

The depositing can comprise dispersing the particles of the islandmaterial in a carrier medium to create a suspension, forming thesuspension into a sheet, causing the sheet to solidify to form a solidsheet, patterning the solid sheet by removing one or more portions ofthe sheet to form a patterned sheet, and overlaying the patterned sheeton the substrate; and the heating can further comprise eliminating thecarrier medium prior to the melting and fusing the particles of theisland material.

The molten globule can have a first wetting angle with a first portionof a surface of the substrate in contact with the molten globule, and asecond wetting angle with a second portion of the surface of thesubstrate, the second portion abutting the first portion, and the secondwetting angle being greater than the first wetting angle.

An area of the substrate in contact with the molten globule can compriseone or more of one or more guiding protrusions, one or more guidingdepressions, and a metallic grid for controlling initiation ofcrystallization as the molten globule is cooled.

The coefficient of thermal expansion (CTE) of the substrate at atemperature within about 20° C. of the melting point of the islandmaterial can match the CTE of the island material at the melting pointof the island material.

The island material can comprise silicon.

The substrate can comprise alumina.

The securing can comprise over-coating the crystalline island and thesubstrate with an over-coating layer to form a stack whereby thecrystalline island is sandwiched between the substrate and theover-coating layer.

When the securing comprises over-coating the crystalline island and thesubstrate with an over-coating layer to form a stack whereby thecrystalline island is sandwiched between the substrate and theover-coating layer, the method can further comprise planarizing thestack to expose a cross-section of the crystalline island.

According to a further aspect of the present specification, there isprovided a method of fabricating a crystalline island of an islandmaterial, the method comprising depositing particles of the islandmaterial on a first substrate, sandwiching the particles of the islandmaterial between the first substrate and a second substrate by placingthe second substrate adjacent the first substrate, heating the firstsubstrate, the second substrate, and the particles of the islandmaterial to melt and fuse the particles to form a molten globule,cooling the first substrate, the second substrate, and the moltenglobule to crystallize the molten globule, thereby forming thecrystalline island of the island material.

The cooling can further comprise one or more of applying a pressurepulse to the molten globule, adding a seed crystal to the moltenglobule, and super-cooling the molten globule.

The first substrate can have a first area being a portion of a surfaceof the first substrate that comes into contact with the molten globule,and the second substrate can have a second area being a portion of asurface of the second substrate that comes into contact with the moltenglobule, and one or more of the first area and the second area cancomprise one or more of one or more protrusions, one or moredepressions, a metallic grid, for controlling initiation ofcrystallization as the molten globule is cooled.

According to a further aspect of the present specification, there isprovided a method of fabricating a crystalline island of an islandmaterial, the method comprising: depositing particles of the islandmaterial abutting a substrate; heating the substrate and the particlesof the island material to melt and fuse the particles to form a moltendisk; cooling the substrate and the molten disk to crystallize themolten disk, thereby securing the crystalline island of the islandmaterial to the substrate; and planarizing at least a portion of thecrystalline island to expose a cross-section of the crystalline island.

According to a further aspect of the present specification, there isprovided a method of fabricating a crystalline island of an islandmaterial, the method comprising: depositing the island material on asubstrate; heating the substrate and the island material, the heatingmelting the island material to form a first molten disk, the heatingalso forming a second molten disk comprising oxygen and the islandmaterial, the second molten disk disposed between the first molten diskand the substrate; and cooling the substrate, the first molten disk, andthe second molten disk to crystallize the first molten disk, therebyforming the crystalline island of the island material.

The method can further comprise planarizing at least a portion of thecrystalline island to expose a cross-section of the crystalline island.

The method can further comprise, after the cooling: over-coating thecrystalline island and the substrate with an over-coating layer to forma stack; and planarizing the stack to expose a cross-section of thecrystalline island.

The method of claim can further comprise, before the depositing: formingan oxide layer on the substrate; and wherein: the depositing cancomprise depositing the island material on the oxide layer; and thesecond molten disk can comprise the oxide layer in a molten state.

The forming the oxide layer can comprise depositing on the substrate theoxide layer comprising an oxide of the island material.

The depositing can comprise depositing the oxide layer according to apredetermined pattern.

The forming the oxide layer can comprise: depositing the island materialon the substrate according to a predetermined pattern; and oxidizing theisland material.

The depositing the island material can comprise one or more of:depositing particles of the island material; and depositing a layer ofthe island material.

The heating can comprise heating the substrate, the island material, andthe oxide layer in a non-oxidizing atmosphere.

The method can further comprise one or more of, before the depositing:polishing the substrate according to a predetermined pattern; androughening the substrate according to the predetermined pattern.

The depositing can comprise depositing the island material on thesubstrate in a shape of a plurality of interconnected nodes, each nodeconnected to one or more other nodes.

The second molten disk can further comprise aluminum originating fromthe substrate.

The heating can comprises heating the substrate and the island materialto at least about 1500° C.

The first molten disk can have a maximum thickness that is at leastabout ten times smaller than the smaller of its maximum length andmaximum width.

According to a further aspect of the present specification, there isprovided a semiconductor device comprising: a substrate; an intermediarydisk disposed on the substrate, the intermediary disk comprising oxygenand an island material; and an island disk disposed on the intermediarydisk, the island disk comprising the island material, the island diskbeing crystalline; wherein: the island material is deposited on thesubstrate; and the intermediary disk is formed by melting and thensolidifying the island material on the substrate.

The substrate can comprise alumina and the island material comprisessilicon.

The intermediary disk can further comprise aluminum originating from thesubstrate.

According to a further aspect of the present specification, there isprovided a method of fabricating a crystalline island of an islandmaterial, the method comprising: depositing the island material on asubstrate; heating the substrate and the island material, the heatingmelting the island material to form a molten corpus, the heating alsoforming a molten disk comprising oxygen and the island material, themolten disk disposed between the molten corpus and the substrate; andcooling the substrate, the molten corpus, and the molten disk tocrystallize the molten corpus to form a crystallized corpus, at least aportion of the crystallized corpus forming the crystalline island of theisland material.

The method can further comprise planarizing at least a portion of thecrystalline island to expose a cross-section of the crystalline island.

The method can further comprise, after the cooling: over-coating thecrystalline island and the substrate with an over-coating layer to forma stack; and planarizing the stack to expose a cross-section of thecrystalline island.

The method can further comprise, before the depositing: forming an oxidelayer on the substrate; and wherein: the depositing comprises depositingthe island material on the oxide layer; and the molten disk comprisesthe oxide layer in a molten state.

The forming the oxide layer can comprise depositing on the substrate theoxide layer comprising an oxide of the island material.

The depositing can comprise depositing the oxide layer according to apredetermined pattern.

The forming the oxide layer can comprise: depositing the island materialon the substrate according to a predetermined pattern; and oxidizing theisland material.

The heating can comprise heating the substrate, the island material, andthe oxide layer in a non-oxidizing atmosphere.

The method can further comprise one or more of, before the depositing:polishing the substrate according to a predetermined pattern; androughening the substrate according to the predetermined pattern.

The substrate can comprise alumina.

The molten disk can further comprise aluminum originating from thesubstrate.

The island material can comprise silicon and the heating can compriseheating the substrate and the island material to at least about 1500° C.

Cooling the molten corpus can form a first solid portion distal from thesubstrate and a second solid portion proximal the substrate, the firstsolid portion separating spontaneously from the second solid portionduring the cooling, the second solid portion forming the crystallineisland.

The depositing can comprise: positioning a template on a surface of thesubstrate, the template comprising a channel having a first end abuttingthe surface and a second end opposite the first end, the surface cappingthe first end; and filling at least a portion of the channel with theisland material.

The method can further comprise: after the cooling, removing thetemplate from the substrate.

After the removing, a first portion of the crystallized corpus canremain on the substrate to form the crystalline island and a secondportion of the crystallized corpus can remain in the channel.

The channel can comprise: a first region proximate the first end, in thefirst region the channel having a first cross-sectional area; and asecond region distal from the first end, in the second region thechannel having a second cross-sectional area, the first cross-sectionalarea different from the second cross-sectional area.

An inner surface of the channel can define a vertex separating the firstregion from the second region.

The template can further comprise one or more further channels, eachfurther channel having a corresponding first end abutting the surfaceand a corresponding second end opposite the corresponding first end, thesurface capping the corresponding first end; and the depositing canfurther comprise filling at least a portion of the one or more furtherchannels with the island material.

The second end can be in communication with a reservoir configured tostore the island material for at least partially filling the channelwith the island material.

The reservoir can be integrally formed with the template.

The reservoir can comprise a crystallization initiator, thecrystallization initiator comprising one or more of a depression intoand an extension from a reservoir surface, the crystallization initiatorconfigured to come into contact with the molten corpus and initiatecrystallization during the cooling.

DESCRIPTION OF THE DRAWINGS

For a better understanding of the various implementations describedherein and to show more clearly how they may be carried into effect,reference will now be made, by way of example only, to the accompanyingdrawings in which:

FIG. 1 depicts a method of fabricating a crystalline island abutting asubstrate, according to non-limiting implementations.

FIG. 2 depicts a top perspective view of crystalline islands on asubstrate, according to non-limiting implementations.

FIG. 3 depicts a top perspective view of a substrate having adepression, according to non-limiting implementations.

FIG. 4 depicts a collection of possible shapes for a depression,according to non-limiting implementations.

FIG. 5 depicts a top perspective view of a substrate having a depressionwithin a depression, according to non-limiting implementations.

FIGS. 6a-d depict a cross-section of a substrate having a depressionwithin a depression at different stages of forming a crystalline islandin the depressions, according to non-limiting implementations.

FIG. 7 depicts a cross-section of a substrate having a depression withina depression, according to non-limiting implementations.

FIG. 8 depicts a cross-section of a substrate having a depression withina depression, according to non-limiting implementations.

FIGS. 9a-c depict a cross-section of a substrate having a through hole,at different stages of forming a crystalline island in the through hole,according to non-limiting implementations.

FIG. 10 depicts a method of depositing particles of an island materialon a substrate, according to non-limiting implementations.

FIG. 11 depicts a cross-section of a crystalline island on a substrate,both over-coated, according to non-limiting implementations.

FIG. 12 depicts a cross-section of a crystalline island on a substrate,both over-coated, according to non-limiting implementations.

FIG. 13 depicts a method of fabricating a crystalline island sandwichedbetween two substrates, according to non-limiting implementations.

FIG. 14 depicts a cross-section of a molten globule sandwiched betweentwo substrates, according to non-limiting implementations.

FIG. 15 depicts a top plan view of an array of pyramids and abuttingcrystal grains on a substrate, according to non-limitingimplementations.

FIG. 16 depicts a cross-section of a molten globule sandwiched betweentwo substrates, according to non-limiting implementations.

FIGS. 17a-d depict in cross-section various stages of fabricating acrystalline island on a substrate, according to non-limitingimplementations.

FIG. 18 depicts a photomicrograph of a crystalline island on asubstrate, according to non-limiting implementations.

FIGS. 19a-c depict, in side elevation cross-sectional views, variousstages of fabricating a crystalline island on a substrate, according tonon-limiting implementations.

FIGS. 20a-c depict, in side elevation cross-sectional views, variousstages of fabricating a crystalline island on a substrate, according tonon-limiting implementations.

FIGS. 21a-b depict, in side elevation cross-sectional views, variousstages of fabricating crystalline islands on a substrate, according tonon-limiting implementations.

FIGS. 22a-b depict, in top plan views, various stages of fabricatingcrystalline islands on a substrate, according to non-limitingimplementations.

DETAILED DESCRIPTION

An implementation of the present invention is reflected in method 100shown in FIG. 1. Method 100 can be used to fabricate a crystallineisland abutting a substrate, on top of or inside a substrate. First, asshown in box 105, island material can be deposited abutting thesubstrate, on or in the substrate. The island material can be inparticulate form. Particles of the island material can be deposited at apredetermined location relative to the substrate, which can include, butis not limited to, deposition on all or a portion of the surface of thesubstrate. Particles of the island material can be deposited as asubstantially pure powder, a powder with some additives or impurities,or as particles of the island material suspended in a carrier medium.Additives can be used to dope, alloy, or otherwise compound the islandmaterial.

Subsequently, as shown in box 110, the substrate and the particles ofthe island material can be heated. The heating can be by conduction,convection, and/or radiative heating, and can be performed in a furnace,kiln, or other suitable heating apparatus known to the skilled person.The heating melts and fuses the particles of the island material to forma molten globule of the island material. When a carrier medium is usedto deposit the island material, the heating can evaporate, burn off, orotherwise eliminate the carrier medium before the melting and fusing ofthe island material particles. The island and substrate materials can bechosen so that the substrate does not melt at the temperature requiredto melt and fuse the particles of the island material.

Subsequently, as shown in box 115, the substrate and the molten globuleare cooled to crystallize the molten globule, thereby securing theresulting crystalline island to the substrate. In some implementations,the islands are secured strongly enough to allow mechanical polishing,or other abrasive processing, of the island to expose a cross-section ofthe island, without dislodging the island from the substrate.

FIG. 2 shows an array of crystalline islands 210 formed on substrate205. Method 100 can be used to fabricate one or any number ofcrystalline islands. When a plurality of islands is formed, they can bearranged in an ordered array, or can be distributed on the substratewithout perceptible periodic ordering. In some implementations, theposition of each island 210 relative to substrate 205 is known to allowfor subsequent processing of the islands.

Islands 210 can be single-crystalline or poly-crystalline.Nano-crystalline and amorphous islands can also be formed. In someimplementations after islands 210 are formed, they are planarized,abraded, or otherwise operated upon so that some material is removedfrom the surfaces thereof to expose a cross-section of each island. Thisexposed cross-section can then be used to fabricate electronic devices,for example when the islands are formed from a semiconductor such assilicon.

When the starting particulate island material has impurities, theprocess of melting and crystallizing set out in method 100 can reducethe impurities inside the crystalline islands by pushing the impuritiestowards the surface of each molten globule as a crystalline latticebegins to form inside the molten globule, a process known as gettering.The lattice then tends to exclude any impurities that would interferewith its ordered arrangement of atoms, thereby excluding at least aportion of the impurities from the inside of the crystal. When thecrystalline islands are poly-crystalline, the impurities are pushed tothe grain boundaries between the crystals.

In some implementations, particles of the island material are depositedonto the substrate by transferring the island material into one or moredepressions defined in the substrate surface. FIG. 3 shows depression310 in substrate 305. In some implementations a plurality of depressionscan be defined, and they can be arranged into an ordered array. Thedepressions can be defined lithographically, or using other means knownto the skilled person, including but not limited to laser ablation orlocalized etching using a photolithographically defined mask.

The depression 310 contains the molten globule, and can serve to moreprecisely locate the molten globule, and the resulting crystallineisland, relative to the substrate 305. In addition, the shape of thedepression can guide the crystallization process by providing nucleationsites for initiating crystallization. In some implementations, thedepression can have one or more vertexes 425 as shown in FIG. 4 inrelation to depressions 405, 410, 415, and 420. The vertex 425 can be atthe end of a taper such as in the case of depression 420, or part of areverse taper, as shown in depressions 405 and 415. A depression canhave multiple vertexes as shown in depressions 410 and 415. Havingmultiple nucleation sites in the form of vertexes 425 increases thelikelihood for the island being poly-crystalline and decreases thelikelihood of formation of a single-crystalline island. Nucleation sitescan also be three-dimensional smaller depressions into or protrusionsfrom the surface of depression 310. Such smaller depressions can haveone or more vertexes (not shown). These vertexes can in turn serve as anucleation site for crystallization of the molten globule.

As shown in FIG. 5, in some implementations, substrate 505 can have alarger and shallower depression 510, within which there can be a smallerand deeper depression 515.

FIGS. 6a-d show a cross-section of a substrate 605, with a largerdepression 610 and a smaller and deeper depression 615 within depression610. As shown in FIG. 6b , particles of the island material 620 can bedeposited to fill both the larger and the smaller depressions 610 and615. As shown in FIG. 6c , when the particles are melted, they fuse toform molten globule 625. The surface tension of the molten globule canpull it into an approximately spherical shape, which is then cooled andcrystallizes to form a crystalline island. As shown in FIG. 6d , thecrystalline island and/or the substrate 605 can be planarized to yield acrystalline island 630 in substrate 605. Depression 610 serves as areservoir for particles of the island material which eventually form themolten globule 625. Depression 615, in turn, serves to locate the moltenglobule 625 and initiate its crystallization using vertexes discussedabove in relation to FIG. 5.

The relative volume of depressions 610 and 615 can determine the size ofthe molten globule. This, combined with the depth of depressions 610 and615 can determine the size of cross-section of crystalline island 630available at different depths of substrate 605. When particles of islandmaterial 620 fuse to form molten globule 625 leaving at least a portionof the volume of depression 610 empty, this empty space can beback-filled after molten globule 625 crystallizes. This back-filling canhelp to create a planar surface.

FIG. 7 shows a side elevation cross-section of a substrate 705 havinglarge depression 710, and small depression 715 within large depression710. As shown in the cross-section, the surface of small depression 715envelops more than half of molten globule 725 (shown in dotted line)thereby physically securing the crystallized molten globule 725 to thesubstrate. Even in implementations where the surface of small depression715 envelops less than half of the molten globule 725, the envelopingcan still contribute to physically securing the crystalline island tothe substrate. The shape of depression 715 is not limited to a portionof a sphere. Any suitable shape can be used. When it is desirable tophysically secure the crystalline island to the substrate, the shape ofdepression 715 can be used whereby the molten globule can flow into theshape, but the solid crystalline island cannot be physically removedfrom depression 715. An example of such shapes for depression 715 can beany shape where the opening of depression 715 is smaller than thelargest dimension of the crystalline island that must pass through theopening in order for the crystalline island to be removed fromdepression 715.

FIG. 8 shows a side elevation cross-section of a substrate 805 having alarge depression 810 and a small depression 815 within large depression810. Small depression 815 has protrusion 820 extending into the space tobe occupied by molten globule 840. Protrusion 820 can have at least onevertex 825. Instead of, or in addition to, protrusion 820, smalldepression 815 can have a further depression 830 in the surface of smalldepression 815. Further depression 830 may have at least one vertex 835.Further depression 830 can also be described as a protrusion of thespace to be occupied by molten globule 840 into small depression 815.Depression 815 may have any number or combination of protrusions 820 andfurther depressions 830. Vertexes 825 and 835 can form a nucleation sitefor initiating the crystallization of molten globule 840. Once moltenglobule 840 is crystallized, protrusion 820 and further depression 830can contribute to physically securing the crystallized island intodepression 815, and in turn securing the crystalline island to substrate805.

The securing means discussed in relation to FIGS. 7 and 8 can also beused in a single-stage depression such as depression 310 shown in FIG.3. In addition to these means of physically securing the crystallineisland to the substrate, surface adhesion can also be used to secure thecrystalline island to the substrate. For example, if the molten globulehas a wetting angle with the substrate of less than about 90°, themolten globule sufficiently wets the substrate surface and contributesto the adhesion of the crystallized island to the substrate surface.

When the particulate island material is in the form of a loose powder,it can be transferred into the depression in the substrate using meansincluding but not limited to: 1) doctor-blading the powder into thedepression; and 2) electrostatically depositing the powder into thedepression using charged pins to pick and then deposit the powder intothe depression.

When the particulate island material is in the form of a suspension ofthe particles in a carrier medium, the suspension can be flowed onto thesubstrate to fill the depression and then squeegeeing the excesssuspension located outside the depression from the surface of thesubstrate. When such a carrier medium is used, during the heating stepit can be evaporated, burnt off, or otherwise eliminated before themelting and fusing of the particulate island material.

In the cooling stage, cooling alone can be sufficient to initiate thecrystallization of the molten globule. Other techniques can be used tofacilitate or more finely control the initiation and progress of thecrystallization. For example, the molten globule can be super-cooledbelow its melting point. Super-cooling can take the form of cooling themolten globule to less than around 300° C. below its melting pointbefore the crystallization starts. Applying a physical impact or shockto the substrate bearing the molten globule can also set offcrystallization. This can also be used when the molten globule issuper-cooled. In addition, the surface of the molten globule can beexposed to different chemical reactants, such as oxygen, to furtherguide the crystallization process. The oxygen can form a thin layer or“skin” on the surface of the molten globule which serves to isolate themolten silicon from the substrate and can serve to increase the surfacetension of the globule.

FIG. 9a shows an implementation where substrate 905 has a through hole910 filled with particulate island material 915. Hole 910 has a firstend 940 and a second end 945. Hole 910 can be filled with particulateisland material 915 using doctor-blading. Hole 910 can also be filledwith a liquid suspension comprising the particulate island materialdispersed in a carrier medium. The suspension can be flowed onto thesubstrate and into hole 910, and then the excess suspension locatedoutside hole 910 can be squeegeed off the surface of the substrate.After filling hole 910, substrate 905 and island material 915 can beheated to melt and fuse the island material into molten globule 935.When such a carrier medium is used, during the heating step it can beevaporated, burnt off, or otherwise eliminated before the melting andfusing of the particulate island material. Optionally, substrate 905 canbe flipped before the heating. For example, the flipping can be usedwhen hole 910 has a closed end, to point the open end of hole 910towards the earth's gravitational force.

FIG. 9b shows molten globule 935 forming a convex meniscus 920 extendingout of the first end 940 of hole 910. Meniscus 920 can form under theforce of gravity. In addition, if surface 925 of substrate 905 adjacentfirst end 940 of hole 910 has a low wetting angle with the moltenglobule 935, this can encourage the molten globule 935 to wet thesurface 925 of substrate 905 and for the meniscus 920 to form and extendout of first end 940 of hole 910. Furthermore, molten globule 930 can beencouraged to extend out of hole 910 and form meniscus 920 if pressureis applied against the molten globule 935 through the second end 945 ofhole 910 to push molten globule 935 out of first end 940 of hole 910. Asshown in FIG. 9c , once molten globule 935 crystallizes, meniscus 920can be polished and/or planarized to expose a cross-section ofcrystalline island 950 and to form a crystalline island 950 in substrate905.

FIGS. 9a-c show one hole 910, but a plurality of holes can be used. Theholes can be arranged in an ordered array. Crystalline islands can besecured to substrate 905 by respective holes, such as hole 910,enveloping and physically securing the respective island 950 and/or thesurface adhesion of the crystalline island 950 to the surface of hole910. Adhesion can be stronger when the wetting angle between the moltenglobule 935 and the surface of hole 910 is smaller than about 90°.

In another implementation (not shown), particles of island material canbe dispersed in a carrier medium to form a suspension. The suspensioncan then be transferred onto the substrate. Next, the substrate and thesuspension can be heated, which can evaporate, burn off, or otherwiseeliminate the carrier medium. The heating can also melt and fuse theparticles of the island material to form a molten globule. The coolingand securing can be carried on as previously described. A wetting angleof less than about 90° between the molten globule and the substrate cancontribute to stronger adhesion between the substrate and crystallineisland and to securing the crystalline island to the substrate.

The suspension can be transferred to the substrate using techniquesincluding, but not limited to, one or more of stamping, screen printing,or inkjet printing of the suspension onto the substrate followingprocedures known in the art. The suspension can also be spin-coated toform a layer on the substrate. This layer can then be lithographicallypatterned to define one or more regions on the substrate where particlesof the island material are present, and other regions where islandmaterial particles are absent.

There may not be any depressions in this implementation. However, thesubstrate surface can be patterned to have areas of higher wetting angleand other areas of lower wetting angle with the molten globule. Themolten globule will tend to form on the areas of lower wetting angle.The patterning of areas with low wetting angle can serve as a means offurther locating the molten globule, and thus the crystalline island onthe substrate. This can be applied to one crystalline island or aplurality of crystalline islands. Methods for patterning a substrate tohave high and low wetting angle areas are well known in the art, and caninclude applying a patterned mask to the surface followed by subjectingthe unmasked areas to chemical modification or deposition of othermaterials, such as SiO₂, on the unmasked areas.

FIG. 10 shows a further implementation of the present inventionreflected in method 1000 for depositing particles of the island materialon the substrate. First, as shown in box 1005, the particles can bedispersed in a carrier medium to create a suspension. Next, as shown inbox 1010, the suspension can be formed into a sheet by spreading or spincoating the suspension or using other methods known in the art. Next, asshown in box 1015, the sheet of the suspension can be transformed into asolid sheet. This can be accomplished by drying, baking, cross-linking,or otherwise solidifying the suspension. Next, as shown in box 1020, thesolid sheet can be patterned by cutting away or removing one or moreportions of the sheet to form a patterned sheet. The pattern can beapplied using a mechanical punch, lithographically, or using other meansknown in the art. Next, as shown in box 1025, the patterned sheet can beoverlaid on the substrate.

At this stage the remaining steps of heating and cooling-and-securingcan be applied as previously described. During heating, the carriermedium can be evaporated, burnt off, or otherwise eliminated beforemelting and fusing of particles of the island material. A wetting angleof less than 90° between the molten globule and the substrate cancontribute to adhesion of the crystalline island to the substrate. Thisimplementation can be used to make a single island or a plurality ofcrystalline islands on the substrate, which can be arranged in anordered array.

In the implementations where there is no depression, initiation of thecrystallization of the molten globule can still be guided andcontrolled. One or more guiding depressions into and/or guidingprotrusion from the substrate surface coming into contact with themolten globule can initiate crystallization. These guiding depressionsand protrusions can have at least one vertex to provide an initiationpoint for the crystallization process.

Alternatively, the shape of the contact area of the molten globule withthe substrate can be controlled to provide an initiation point forcrystallization. By patterning the relatively low and high wetting angleareas on the substrate, the molten globule can be made to wet or contactthe substrate along a patterned lower wetting angle shape while avoidingthe higher wetting angle areas of the substrate. The shape of the lowwetting angle area can be any of the shapes discussed above in relationto FIG. 4. The shape can have at least one vertex to provide aninitiation point for the crystallization.

Another means of controlling initiation of crystallization can bedepositing a metallic grid on the substrate, at least over the areas ofthe substrate that come into contact with the molten globule. Thedeposited metal can act as an initiation point for the crystallizationof the molten globule. The grid can be made of other materials, such asrefractories or Ni. The deposited material can have other shapes such asdots or other patterns of deposited material that may not constitute agrid.

In other implementations, after the crystalline islands form, theislands and the substrate can be over-coated. FIG. 11 shows across-section of substrate 1105 and crystalline island 1110 over-coatedwith layer 1115. This assembly forms a stack where the crystallineisland 1110 is sandwiched between substrate 1105 and cover-coating layer1115. The stack can then be planarized to remove some of the materialcomprising the stack and expose a cross-section of the crystallineisland.

The over-coating layer 1115 can be deposited using any suitable physicalor chemical deposition method including but not limited to spin coatingor electrostatically-applied powder coating. The layer can be a thinlayer, such as layer 1115 in FIG. 11, or can be a thicker layer such aslayer 1215 shown in cross-section in FIG. 12. When an over-coating layeris used, it can physically secure the crystalline island to thesubstrate by sandwiching the crystalline island between the substrateand the over-coating layer, as shown in FIGS. 11 and 12. This securingaction of the over-coating layer can contribute to securing to thesubstrate crystalline islands having a large wetting angle, andtherefore small contact area, with the substrate. Large wetting anglescan, for example, be angles greater than about 90°. Over-coating can beapplied to a plurality of islands on a substrate.

Another implementation of the present invention is reflected in method1300 shown in FIG. 13. Box 1305 shows depositing particles of an islandmaterial on a first substrate. Particles can be deposited as loosepowder or in a suspension as described above. The deposition can bepatterned and/or at specified locations relative to the substrate. Next,as shown in box 1310, the particles of the island material can besandwiched between the first substrate and a second substrate placedadjacent the first substrate. Next, as shown in box 1315, the substrateand the particles can be heated to melt and fuse the particles into amolten globule, without melting the first substrate or the secondsubstrate.

When the particles are deposited as a suspension in a carrier medium,the heating step can evaporate, burn off, or otherwise eliminate thecarrier medium before melting and fusing the particles. Next, as shownin box 1320, the substrates and the molten globule can be cooled tocrystallize the molten globule, thereby forming a crystalline island.

The sandwiching, described in box 1310, and the heating described in box1315 can be performed in the opposite order, i.e. the molten globule canform before it is sandwiched between the first and the secondsubstrates. FIG. 14 shows molten globule 1415 sandwiched between firstsubstrate 1405 and second substrate 1410.

When the wetting angle between molten globule 1415 and both firstsubstrate 1405 and second substrate 1410 is large, the crystallizedisland can adhere more weakly to the substrate. The weak adhesion canfacilitate removing the crystallized island from the substrate to form afree-standing wafer.

Features on the surfaces of one or both of the first and secondsubstrates can be used to initiate crystallization of the moltenglobule. FIG. 15 shows an array of pyramids 1505 that can be formed onthe region of the substrate that comes into contact with the moltenglobule. Pyramids 1505 can protrude from the substrate surface or formdepressions into the substrate surface. Instead of a pyramid, othershapes can be used, such as a cone. These shapes can have a vertex, asdoes pyramid 1505, to act as an initiation site for the crystallizationof the molten globule.

Each pyramid 1505 initiates crystallization to form grain 1510, whicheventually abuts upon neighboring grains at grain boundaries 1515. Usingthis method, a molten globule can be patterned into a poly-crystallineform. As discussed above, during crystallization at least some of theimpurities in the molten globule can be pushed towards the grainboundary regions. This process can leave the central region of grain1510 with relatively fewer impurities yielding a higher quality crystalfor post-processing, such as device fabrication on grain 1510. Thisprocess can isolate the grain boundaries to regions where devices willnot be fabricated in subsequent processing. Although FIG. 15 shows fourgrains 1510 of uniform size, grains 1510 can also be of different sizes.

FIG. 16 shows a cross-section of molten globule 1615 sandwiched betweentwo substrates 1605 and 1610. As discussed above, one or both of thesubstrates can include depressions or protrusions to guide thecrystallization process. Substrate 1605 can have protrusions 1620 incontact with molten globule 1615. Substrate 1610, in turn, can havedepressions 1625 in contact with molten globule 1615. The protrusions1620 and depressions 1625 can be of different shapes and any numbers ofthem can be used in/on one or both of first substrate 1605 and secondsubstrate 1610. In addition or instead of protrusions and depressions, agrid or array of a metal or other material deposited on one or both ofthe first and the second substrate where those substrates come intocontact with the molten globule can also be used to initiate and guidethe crystallization process.

The cooling as shown in box 1320 of FIG. 13 can also includesuper-cooling the molten globule. The super-cooling can include coolingthe molten globule to a temperature below about 300° C. below itsmelting point before crystallization begins. A pressure pulse ormechanical impact can also be applied to the molten globule in asuper-cooled state or otherwise. A seed crystal can also be added to themolten globule in a super-cooled state or otherwise.

In some implementations, the coefficient of thermal expansion (CTE) ofthe substrate at a temperature within about 20° C. of the melting pointof the island material can be matched to the CTE of the island materialat the melting point of the island material. This matching of CTE canreduce stresses between the island material and the substrate as eachone cools and contracts. Lower stresses can facilitate making of higherquality crystals with fewer defects, and can improve the adhesion of thecrystalline island to the substrate.

The island material can include, but is not limited to, semiconductors.Such semiconductor can include, but are not limited to, silicon. Thesubstrate material can include, but is not limited to, silica, alumina,sapphire, niobium, molybdenum, tantalum, tungsten, rhenium, titanium,vanadium, chromium, zirconium, hafnium, ruthenium, osmium, iridium, andcombinations and alloys of these materials.

The substrate can also be a ceramic or glasses with sufficiently highmelting or softening temperatures. The substrate can also be aHigh-Temperature Co-fired Ceramic (HTCC). HTCC can be worked andmechanically patterned in its green phase. When the island material issilicon, alumina can be a relatively higher wetting angle material andsilica a relatively lower wetting angle material.

In some implementations, the molten globule of the island material canhave a flattened or disk shape. For example, referring to FIG. 1, atstep 110 heating the substrate and the particles of the island materialcan melt and fuse the particles to form a molten disk and/or a moltendisk-shaped or flattened globule. At step 115, the cooling can thensolidify and crystallize the molten disk, thereby securing thedisk-shaped crystalline island of the island material to the substrate.The securing can comprise the crystalline island adhering directly tothe substrate. In addition and/or instead, the securing can comprise thecrystalline island adhering indirectly to the substrate by adhering toany intermediate and/or interfacial layer secured directly to thesubstrate. In some implementations, some portions of the crystallineisland can adhere directly to the substrate material while otherportions can adhere to an interfacial and/or intermediate layer coveringat least a portion of the substrate. In some implementations, after thecooling step, at least a portion of the crystalline island can beplanarized to expose a cross-section of the crystalline island.

FIGS. 17a-d show various steps of an exemplary method for fabricatingcrystalline islands that are disk shaped and/or have a flattened shape.FIG. 17a shows island material 1710 being deposited on a substrate 1705.Substrate 1705 can be similar to the other substrates described herein,and island material 1710 can likewise be similar to other islandmaterials described herein.

While FIG. 17a shows island material 1710 deposited as a heap or mound,it is contemplated that island material 1710 can be deposited in anyother suitable manner. For example, island material 1710 can bedeposited in powder form. When island material 1710 is deposited inpowder form, the powder can be deposited through a screen to form onemound or an array of mounds of powder at predetermined positions onsubstrate 1705. The size of the openings in the screen can determine theshape and size of the mounds of powder on substrate 1705.

In other implementations, island material 1710 can be deposited as alayer of material on the substrate. In yet other implementations, theisland material 1710 can be suspended in a carrier medium, and thesuspension can be deposited on substrate 1705. Such layers and/orsuspensions of island material 1710 can be patterned on substrate 1705and/or deposited at predetermined positions on substrate 1705. In someimplementations, island material 1710 can be printed on substrate 1705.

Referring to FIG. 17b , once island material 1710 is deposited onsubstrate 1705, island material 1710 and substrate 1705 can be heated tomelt island material 1710 to form a first molten disk 1715. The heatingcan also form a second molten disk 1707 disposed between first moltendisk 1715 and substrate 1705. Second molten disk 1707 can compriseoxygen and the island material.

The molten disks can have any generally flattened shape, including butnot limited to, a saucer, a pancake, a wafer, a platelet, a discus, asheet, and/or an oblate shape. The molten disks can have a maximumthickness that is at least about ten times smaller than the smaller oftheir maximum length and maximum width. In some implementations, moltendisks can have a maximum thickness that is at least about five timessmaller than the smaller of their maximum length and maximum width. Inother implementations, molten disks can have a maximum thickness that isat least about two times smaller than the smaller of their maximumlength and maximum width. The first and second molten disks can belargely or entirely immiscible, thereby remaining largely or entirelyphase-separated in the molten state. In addition, second molten disk1707 can have a higher density in the molten state, thereby remaining,under the force of gravity, between substrate 1705 and first molten disk1715.

After the heating, substrate 1705, first molten disk 1715 and secondmolten disk 1707 can be cooled to solidify and/or crystallize firstmolten disk 1715 to form the crystalline island of the island material.In this process, second molten disk 1707 can also solidify to form anoxide disk. The crystalline island can also have a disk like (orgenerally flattened) shape similar to the shape of first molten disk1715.

The crystalline island can be single, poly, and/or nano crystalline. Insome implementations, after forming the crystalline island, at least aportion of the crystalline island can be planarized to expose across-section of the crystalline island. In some implementations, theisland can be mechanically (and/or chemo-mechanically) planarizedwithout becoming detached from the substrate. This can be made possiblebecause the crystalline island can adhere strongly to the oxide disk,which in turn can adhere strongly to the substrate.

Several factors can contribute to the strong adhesion of the crystallineisland to the substrate. One such factor can be the relatively smallwetting angle and thereby relatively large contact area between thecrystalline island and the oxide disk and also between the oxide diskand the substrate. If either one of the crystalline island and the oxidedisk were to have a large wetting angle, and thereby a tendency toball-up into a near-spherical shape, there would be much smaller contactarea, and weaker adhesion, between the crystalline island, the oxidedisk, and the substrate. Such balled-up, near-spherical crystallineislands can adhere only weekly to the substrate such that they wouldbecome detached from the substrate during planarization, such asmechanical and/or chemo-mechanical planarization.

Another factor contributing to the strong adhesion can be porosity ofthe substrate, which can also increase the contact surface area betweenthe substrate and the oxide disk and/or crystalline island in contactwith the substrate. Yet another factor contributing to the strongadhesion of an alumina substrate to an oxide layer comprising siliconoxide is that often alumina substrates comprise some glass mixed in withthe aluminum oxide. Since most glass comprises silicon oxide, the glasscomponent of the alumina substrate can adhere strongly to the oxide diskwhich can also comprise silicon oxide.

In some implementations, as shown in FIG. 17c , after the cooling thecrystalline island and substrate 1705 can be over-coated with anover-coating layer 1720 to form a stack. This over-coating layer 1720can be similar to other over-coating layers described herein. As shownin FIG. 17d , after the over-coating, the stack can be planarized toremove portions of the over-coating layer and the crystalline island,and as a result expose a cross-section 1725 of the crystalline island.

In some implementations, second molten disk 1707 can be formed whenisland material 1710 is heated on substrate 1705 in the presence ofoxygen. For example, oxygen can be present in gaseous form if islandmaterial 1710 and substrate 1705 are headed in an air atmosphere. Insome implementations, second molten disk 1707 can comprise a moltenoxide of the island material which phase separates from first moltendisk 1715 comprising molten island material.

The presence of the second molten disk, combined with temperatures inexcess of island material's melting point during the heating step, canallow the molten globule of the island material to spread into a disk,instead of balling-up into a near-sphere under surface tension forces.Such temperatures can also reduce the viscosity of the molten islandmaterial, thereby promoting the ability of the molten island material tospread into a molten disk. In addition, the second molten disk can allowthe first molten disk to cool and solidify into a disk and/or flattenedshape. Without the second molten disk, as the temperature is reduced toapproach the melting point of the island material, the decreasingtemperature can cause an increase in the surface tension of the moltenisland material, thereby causing it to ball-up into a near-sphere.

Moreover, the second molten disk can allow the first molten disk tocrystallize into a crystalline island while minimizing interference withcrystal formation during the cooling step due to lattice and/or CTEmismatches between the island material and the substrate. Reducing theseinterferences and/or mismatches can also strengthen the mechanicaladhesion of the crystalline island, via the oxide disk, to thesubstrate.

In one particular example, crystalline islands of silicon can befabricated on an alumina substrate. The alumina substrate can compriseAlumina Ceramic Substrate 10×10×0.5 mm, one side polished(ALCeramic101005S1) sold by MTI Corporation. Particles of silicon can bedeposited as heaps onto the alumina substrate. The deposition can becarried out using a screen with holes having a diameter of about 1 mm.Then the alumina substrate and the heaps of silicon island material canbe heated in an air atmosphere, according to the temperature profilesummarized in Table 1 below:

TABLE 1 Temperature Profile Ramp rate Level temp Dwell time Step (°C./min) (° C.) (min) 1 3 1000 0.1 2 10 1600 60 3 10 1200 0.1 4 5 500 0.1

As can be seen in Table 1, the maximum temperature is 1600° C., which isin excess of the melting point of silicon, which is 1414° C. In otherimplementations, the maximum temperature can be at least about 1500° C.In the case of silicon deposited on alumina, flattening of the firstmolten globule into the first molten disk, and consequent formation of aflattened/disk shaped crystalline islands has not been observed attemperatures below 1500° C. At the conclusion of step 4 in thetemperature profile, the heater can be turned off, and the sample can beallowed to cool further to facilitate subsequent handling.

In addition, while the dwell time at 1600° C. is 60 minutes, dwell timesas short as at least 5 minutes at 1600° C. can cause flattening of themolten island material into the first molten disk, and consequentformation of a flattened/disk shaped crystalline island. Generally, insome implementations, the maximum temperature can be at least about 86°C. above the melting point of the island material. In otherimplementations, the maximum temperature can be at least about 186° C.above the melting point of the island material.

FIG. 18 shows a top plan view optical micrograph of a flattened/diskshaped island 1810 of silicon formed on an alumina substrate 1805,fabricated using the temperature profile summarized in Table 1. When theisland material comprises silicon and the substrate comprises alumina,in some implementations the second molten disk, and the oxide disk intowhich it solidifies, can comprise aluminum in addition to oxygen andsilicon. In some implementations, this aluminum can originate from thealumina substrate. In general, in some implementations, the secondmolten disk can comprise one or more elements originating from thesubstrate.

In the implementations described above, the second molten disk, and theoxide disk into which it solidifies, are formed by heating in thepresence of oxygen the island material deposited on the substrate. It isalso contemplated that in some implementations an oxide layer can beformed on the substrate before the island material is deposited on thesubstrate. This oxide layer can comprise the island material and oxygen.In other implementations, the oxide layer can also comprise additionalmaterials including, but not limited to, elements originating from thesubstrate.

In implementations where the oxide layer is initially formed on thesubstrate, the depositing step can comprise depositing the islandmaterial on the oxide layer. In such implementations, the second moltendisk can comprise the oxide layer in a molten state. In someimplementations, forming the oxide layer can comprise depositing on thesubstrate the oxide layer, which can comprise an oxide of the islandmaterial. In some implementations, the depositing can be according to apredetermined pattern, for example using a mask, printing, lithography,and the like.

In other implementations, forming the oxide layer can comprisedepositing an initial amount of the island material on the substrate andthen oxidizing this initial amount of the island material to form theoxide layer. The deposition of the initial amount of the island materialcan be according to a predetermined pattern. The initial amount of theisland material can be deposited as particles or as a layer of theisland material. For example, in the case of silicon island material,the initial amount can be deposited as silicon particles/power and/or asa layer of amorphous, nano-crystalline, and/or poly-crystalline silicon.

In implementations where the oxide layer is formed on the substrateprior to depositing the island material on the oxide layer, no furtheroxide needs to be formed during the heating step. As such, the heatingcan be performed in a non-oxidizing atmosphere. For example, theatmosphere can be substantially oxygen-free. In some implementations, aninert atmosphere can be used during the heating. For example, theheating can be performed in an argon atmosphere.

In some implementations, the island material can be deposited on thesubstrate in a predetermined pattern. This pattern can comprise aninterconnected and/or contiguous pattern, including but not limited tothe shape of a plurality of interconnected nodes, with each nodeconnected to one or more other nodes. This can allow for thecrystallization of the molten island material to start at one, or a few,nucleation sites in the pattern and then proceed throughout the pattern.This mode of crystallization can allow subsets of the crystallineislands to have similarly oriented crystal lattices. In someimplementations, a single crystal can propagate through all orsubstantially all of the interconnected pattern of the island material.All the means and methods described herein for initiating and/orcontrolling crystallization can be used to initiate and/or control thecrystallization of the molten island material deposited in thepredetermined pattern.

While the foregoing describes disks of molten material, it iscontemplated that the molten material can be of any generally flattenedshape, depending on the pattern according to which the island materialand/or the oxide layer is deposited and/or formed on the substrate. Forexample, if the island material is deposited on the substrate accordingto a pattern of interconnected nodes, then the heating step can producea generally flattened layer of molten island material also generally inthe shape of interconnected nodes. The molten oxide layer can also begenerally in the shape of interconnected nodes. Moreover, once the layerof molten island material crystallizes, the resulting crystallizedisland material can also comprise a generally flattened layer in theshape of interconnected nodes.

While the above description relating to FIGS. 17a-d and FIG. 18 refersto the molten island material being in the shape of a first molten disk,it is contemplated that the molten quantity of the island material neednot be in a flattened or disk shape, and can form into the shape of anybounded and contiguous quantity of molten material, which can have anyshape dictated by its material properties such as viscosity and surfacetension and by external factors such as gravity and the shape of thecontainer/substrate supporting the molten material. Such shapes can begenerally and/or colloquially described as a corpus, body, quantity,glob, blob, globule, dab, and the like. These general and/or colloquialdescriptions also include flattened and/or disk shaped quantities of themolten island material.

As such, the molten globule and/or the molten disk of the islandmaterial can also be described as a molten corpus of the islandmaterial. When the molten corpus is cooled it can solidify to form acrystallized corpus. All or a portion of this crystallized corpus can,in turn, form the crystalline island of the island material.

In some implementations, if the amount and/or volume of the depositedisland material is large relative to the footprint of the depositedmaterial on the substrate, then upon heating the volume of the moltenisland material may be too large for the molten island material to forma flattened or disk shape. Such a larger quantity of molten islandmaterial can form a molten corpus that is more rounded, i.e. has amaximum thickness which, relative to its maximum length and maximumwidth, is larger than the maximum thickness of a flattened or disk shapewith the same maximum length and/or maximum width.

Moreover, in some implementations, such a more rounded molten corpus canbegin to cool and crystallize from a point distal from the substrate. Asthe wave of crystallization propagates from this point towards thesubstrate, the wave can create strain and/or other forces internal tothe molten corpus. Other examples of other such forces can include, butare not limited to, strain caused by the differences between thecoefficient of thermal expansion (CTE) of the crystalized corpus (and/orcooling and crystallizing molten corpus) and the CTE of the solidifiedoxide disk (and/or cooling and solidifying molten oxide disk).

These forces can cause a first solid portion distal from the substrateto spontaneously separate or pop-off from the remainder of the coolingand solidified molten corpus that remains attached to the substrate.This remainder, in turn, can form a second solid portion that isproximal to the substrate and forms the crystalline island. In such ascenario, the crystallized corpus is divided into two portions: thefirst solid portion and the second solid portion. In someimplementations, some or all of the separated first solid portions canbe collected and recycled/reused as island material.

While the above description of the wave of crystallization refers to thewave starting at a point distal from the substrate, it is contemplatedthat based on temperature profiles and/or cooling profiles of thecomponents surrounding the molten corpus, in some implementations thewave of crystallization can propagate in the opposite direction, i.e.from a point proximal the substrate towards a point distal from thesubstrate. In other implementations, the wave of crystallization canpropagate in a different direction dictated by the temperature and/orcooling profile of the components surrounding the molten corpus.

So long as the wave and/or pattern of crystallization and/or coolingcreates sufficient internal strains/forces between the first solidportion and the second solid portion, these two solid portions canspontaneously separate from one another. It is also contemplated thatthese internal forces may build up as the molten corpus solidifies andcontinues to cool, and the internal forces can become large enough tocause spontaneous separation when the crystallized corpus is in thecooling phase.

In some implementations, the surface roughness of the substrate can bepatterned in order to guide where on the substrate the molten oxidelayer and/or the molten island material layer form. For example, thesubstrate can be polished and/or roughened according to a predeterminedpattern. Regions of the substrate with different surface roughnesses canhave different wettability by the molten oxide and/or molten islandmaterial. In addition, regions of the substrate with different surfaceroughnesses can adhere to the oxide layer with different mechanicalstrengths.

Following the method depicted in FIGS. 17a-d , and/or the other similarmethods described herein, a semiconductor device can be fabricated. Sucha device comprises a substrate and an intermediary disk disposed on thesubstrate. The intermediary disk can comprise oxygen and an islandmaterial. The intermediary disk can comprise the oxide disk. Thesemiconductor device also comprises an island disk disposed on theintermediary disk. The island disk can comprise the island material, andcan be crystalline. The island material can be formed separately fromthe substrate and then deposited on the substrate. The intermediary diskcan be formed by melting and then solidifying the island material on thesubstrate. As discussed above, in implementations where the oxideintermediary disk is formed during the heating step, the heating and/ormelting can be performed in the presence of oxygen and at a maximumtemperature exceeding the melting point of the island material.

In addition, while the above description refers to intermediary andisland disks, it is contemplated that the intermediary oxide and/or thecrystallized island material can be in any layer-like or otherwiseflattened shape or configuration. The island disk can have a maximumthickness that is at least about ten times smaller than the smaller ofits maximum length and maximum width. In some implementations, theisland disk can have a maximum thickness that is at least about fivetimes smaller than the smaller of their maximum length and maximumwidth. In other implementations, the island disk can have a maximumthickness that is at least about two times smaller than the smaller oftheir maximum length and maximum width.

In some implementations, the substrate can comprise alumina and/or theisland material can comprise silicon. Moreover, in some implementationsthe intermediary disk can also comprise aluminum. This aluminum canoriginate from the alumina substrate.

The planarized cross-sections of the crystalline islands can be used tomake electronic devices, such as transistors or other circuitcomponents. As such, the methods and devices described herein can beused in backplanes for active matrix displays such as OLED displays, inelectro-optical detector arrays such as X-ray detectors, and infabricating certain integrated circuits such as those used in amplifiersand op-amps.

When multiple crystalline islands are formed on a substrate, and/or whenmultiple electronic devices are fabricated on a given planarizedcross-section, the islands and/or the devices respectively can beappropriately singulated to provide individual crystalline islandsand/or electronic devices respectively. When separated crystallineislands (or arrays of crystalline islands) are used to make separatedisplays and/or detectors, those displays and/or detectors can be tiledtogether to form a larger tiled display and/or detector.

In some implementations, depositing the island material on the substratecan comprise positioning a template on a surface of the substrate, thetemplate comprising a channel having a first end abutting the surfaceand a second end opposite the first end. The surface can cap and/orcover the first end. At least a portion of the channel can be filledwith the island material, either before or after the template ispositioned on the substrate. FIG. 19a shows a side elevationcross-section of a template 1915 positioned on a surface 1910 of asubstrate 1905. Template 1915 comprises a channel 1920 having a firstend 1925 abutting surface 1910 of substrate 1905 and a second end 1930opposite first end 1925. A portion of channel 1920 is filled with islandmaterial 1935.

Substrate 1905 can be similar to substrate 1705 and/or the othersubstrates described herein. Template 1915 can be formed of hightemperature co-fired ceramic and/or any other suitable material thatdoes not melt during the operational temperatures described herein andwould not react with and/or otherwise contaminate the crystalline islandin a manner that would render the crystalline island unsuitable forforming electronic devices. Channel 1920 can have any suitable shapeand/or cross-section, including but not limited to, a cylindrical shapeand/or a circular cross-section.

Once template 1915 and island material 1935 therein are on substrate1905, they can all be heated and then cooled in a manner similar to thatdescribed above, which heating can cause island material 1935 to melt toform a molten corpus and then solidify to form crystallized corpus 1940,as shown in FIG. 19b . An oxide layer 1945 can form between substrate1905 and crystallized corpus 1940. Formation of crystallized corpus 1940and oxide layer 1945 can be similar to the formation of the crystallineisland and the oxide disk described above in relation to FIGS. 17a-d andFIG. 18.

At this point, a portion of template 1915 and crystallized corpus 1940can be removed, e.g. by chemo and/or mechanical planarization, to exposea crystalline island of the island material. This planarization processis not shown in the figures; however, similar planarization steps areshown in FIGS. 6d and 17d and described in relation thereto. In someimplementations, template 1915 can be selectively etched or otherwiseselectively removed, and then crystallized corpus 1940 can beplanarized.

Moreover, referring to FIG. 19c , in some implementations, template 1915can be removed by lifting template 1915 off from substrate 1905 tocreate a crystalline island of the island material secured to substrate1905. In this process, a first portion 1950 of crystallized corpus 1940can remain on and/or secured to substrate 1905 to form the crystallineisland and a second portion 1955 of crystallized corpus 1940 can remainin channel 1920. As template 1915 is lifted off and oxide layer 1945 andfirst portion 1950 remain on substrate 1905, oxide layer 1945 and firstportion 1950 can leave a corresponding space 1960 in channel 1920. Space1960 can be proximate first end 1925 (shown in FIG. 19a ) of channel1920.

In implementations where template 1915 is lifted off, steps can be takento reduce stiction between oxide layer 1945 and first portion 1950 andthe surface of channel 1920. For example, and without limitation, thematerial of template 1915 can be selected to reduce the wettability ofthe channel surface by the molten oxide material and/or the moltenisland material. In addition, morphology (e.g. roughness) and/orcomposition of the surface of channel 1920 can be selected to similarlyreduce wettability of the channels surface by the molten oxide materialand/or the molten island material. Similar steps, and/or other suitablesteps, can be taken to reduce stiction between template 1915 andsubstrate 1905.

Several different methods can be used to cause and/or facilitate theseparation of first portion 1950 from second portion 1955: in someimplementations, second portion 1955 can spontaneously separate and/orpop-off from first portion 1950 in a manner similar to the spontaneousseparation described above. In other implementations, the surface ofchannel 1920 can comprise a stress concentrator and/or separationinitiator (not shown in FIGS. 19a-c ) to facilitate the separation offirst portion 1950 from second portion 1955.

The stress concentrator and/or separation initiator can comprise adepression into and/or a projection from the surface of channel 1920.Such a stress concentrator can concentrate internal stresses/strains incrystallized corpus 1940 at the point between first portion 1950 andsecond portion 1955. Such a concentration can facilitate the spontaneousseparation described above. Instead and/or in addition, when acting as aseparation initiator, such a depression and/or projection can allow amechanical and/or acoustic impulse applied to template 1915 to crackcrystallized corpus 1940 and separate first portion 1950 from secondportion 1955. In some implementations, a thermal shock (i.e. rapidchange of temperature) can be used instead of and/or in addition to themechanical and acoustic impulses.

In some implementations, the stress concentrator and/or separationinitiator can comprise a region or circumferential band where thesurface of channel 1920 is roughened or made jagged. In otherimplementations, the stress concentrator and/or separation initiator cancomprise a pattern of pin-shaped projections projecting from the surfaceof channel 1920.

While FIGS. 19b-c depict formation of oxide layer 1945, it iscontemplated that under different operational conditions, e.g. lowermaximum temperatures and/or in the absence of oxygen, there may be nooxide layer, and crystallized corpus 1940 and first portion 1950 can bedirectly in contact with substrate 1905.

Moreover, while FIG. 19c depicts a smooth border between first portion1950 and second portion 1955, it is contemplated that this border can beuneven and/or rough. In such a case, first portion 1950 can beplanarized to expose a flat and/or smooth cross-section that can besuitable for formation of electronic devices.

In addition, as there is island material remaining (in the form ofsecond portion 1955) in channel 1920 of the lifted-off template 1915,this template can be reused in another heating/cooling cycle to formanother crystalline island without necessarily the need to addadditional island material to channel 1920. In this manner, template1915 can function as a multi-use and/or reusable “print head” forforming and/or “printing” multiple crystalline islands on one or moresubstrates. Moreover, in some implementations template 1915 can comprisemultiple channels, which can have various shapes and/or arrangements.Such a multi-channel template can be used to form and/or “print”crystalline islands of correspondingly different shapes and/orarrangements on one or more substrates.

Turning now to FIGS. 20a 20b, and 20c , these figures are generallysimilar to FIGS. 19a-c , with the difference being that channel 2020 isshaped differently than channel 1920. FIG. 20 a shows in side elevationcross-section substrate 1905, and a template 2015 having a channel 2020,which channel 2020 in turn has first end 2025 and second end 2030opposite first end 2025. Channel 2020 is at least partially filled withisland material 1935. Channel 2020 comprises a first region 2065proximate first end 2025 and a second region 2070 distal from first end2025. In first region 2065 channel 2020 has a cross-sectional area thatis larger than the cross-sectional area of channel 2020 in second region2070. In other implementations, channel 2020 can have a cross-sectionalarea and/or shape that is different in any other suitable manner betweenfirst region 2065 and second region 2070.

Inner surface of channel 2020 defines a vertex 2075 separating firstregion 2065 from second region 2070. Vertex 2075 can act as the stressconcentrator and/or separation initiator described in relation to FIGS.19b-c above. While in FIG. 20a vertex 2075 is shown as a right angle, itis contemplated that the vertex can be any other sharp or angularprojection from and/or depression into the surface of channel 2020.

Referring now to FIGS. 20b-c , as the cross-sectional area of channel2020 is larger in first region 2065, template 2015 can form oxide layers2045 and first portions 2050 (which from the crystalline island) thathave a larger area without the need for having a larger cross-sectionalarea along the full length of channel 2020. After the heating and thecooling, the island material 1935 in channel 2020 melts and thensolidifies to form crystallized corpus 2040 and oxide layer 2045.

Subsequently, template 2015 can be removed and/or lifted off, wherebyfirst portion 2050 of crystallized corpus 2040 can remains on and/orsecured to substrate 1905 and a second portion 2055 of crystallizedcorpus 2040 can remain inside channel 2020. As template 2015 is liftedoff and oxide layer 2045 and first portion 2050 remain on substrate1905, oxide layer 2045 and first portion 2050 can leave a correspondingspace 2060 in channel 2020.

Turning now to FIGS. 21a and 21b , these figures depict select steps ina method of fabricating a crystalline island similar to the stepsdepicted in FIGS. 19a-c and 20a-c , with the main difference being thatthe shape of template 2115 is different from the shape of templates 1915and 2015. Moreover, the process depicted in FIGS. 21a-b can also have astep/state similar to those shown in FIGS. 19b and 20b ; however, thisstep/state is not shown in FIGS. 21a-b for simplicity.

The first difference between templates 1915 and 2015 and template 2115is that template 2115 comprises two channels 2120 a and 2120 b. As such,template 2115 can be used to form and/or “print” two crystalline islandsat a time. While FIGS. 21a-b show template 2115 as comprising twochannels 2120 a,b, it is contemplated that in other implementationstemplate 2115 can comprise one or any number of channels, which channelscan be arranged in any suitable way.

The second difference between templates 1915 and 2015 and template 2115is that template 2115 comprises walls 2130 extending from surface 2125of template 2115 in a direction away from a longitudinal direction ofchannels 2120 a,b. In other words, walls 2130 can extend from surface2125 of template 2115 in a direction away from the side and/or surfaceof template 2115 that is configured to come into contact and/or abutsubstrate 1905. Walls 2130 can be formed integrally with template 2115.Walls 2130 and surface 2125 can cooperate to form a reservoir 2135configured to store island material 1935. Reservoir 2135 can be incommunication with ends of channels 2120 a,b such that island material1935 stored in reservoir 2135 can be transferred into and used to atleast partially fill one of more of channels 2120 a,b. Island material1935 stored in reservoir 2135 can allow template 2115 to be used forforming/“printing” a relatively larger number of crystalline islandsbefore additional island material 1935 needs to be supplied from asource external to template 2115.

Referring now to FIG. 21b , after a cycle of heating to melt islandmaterial 1935 to form a molten corpus and then cooling the molten corpusto form a crystallized corpus and an oxide layer 2145 a, template 2115can be lifted off from substrate 1905. Upon the lifting off, a firstportion 2150 a of the crystallized corpus can remain secured tosubstrate 1905 to form the crystalline island, while a second portion2155 of the crystallized corpus can remain in template 2115. As template2115 is lifted off and oxide layer 2145 a and first portion 2150 aremain on substrate 1905, oxide layer 2145 a and first portion 2150 acan leave a corresponding space 2160 a in channel 2120 a. While notnumbered or described (for the sake of brevity), as shown in FIG. 21b asimilar oxide layer, first portion, and space are also formedcorresponding to channel 2120 b.

While FIGS. 21a-b show reservoir 2135 having a particular geometry andbeing integrally formed with template 2115, it is contemplated that inother implementations the reservoir can have any other suitabledimensions, shape, and/or capacity. In addition, it is contemplated thatin other implementations reservoir 2135 can be formed using walls and/orother components that are not integrally formed with template 2115, butrather are secured and/or connected to template 2115 and/or to channels2120 a,b. In yet other implementations, the reservoir can comprise aseparate container in communication with channels 2120 a,b to allow theisland material to be transferred from the reservoir to channels 2120a,b.

Turing now to FIGS. 22a and 22b , top plan views of a template 2215 areshown resting on substrate 1905. Template 2215 can be generally similarin functionality to template 2115 with one difference being thattemplate 2215 comprises four channels 2220 a,2220 b,2220 c, and 2220 d.A circumferential wall 2225 extends from a face of template 2215 in adirection opposite the longitudinal direction of channels 2220 a-d. Inother words, wall 2225 extends from the top face of template 2215 beingthe face that is opposite the face configured to abut substrate 1905,and wall 2225 extends in a direction generally away from the faceconfigured to abut substrate 1905.

Wall 2225 cooperates with the top face of template 2215 to form areservoir 2230 for storing island material 1935. Reservoir 2230 is incommunication with channels 2220 a-d such that island material 1935 canbe transferred from reservoir 2230 into channels 2220 a-d to at lastpartially fill these channels. Walls 2225 have an inner surface 2235which can come into contact with island material 1935 stored inreservoir 2230. Surface 2235 can comprise a notch 2240 having a sharpvertex.

During the heating, notch 2240 can come into contact with the portion ofthe molten corpus in reservoir 2230, which molten corpus would becontiguous between reservoir 2230 and channels 2220 a-d. During thecooling, notch 2240 and/or its vertex can act as a crystallizationinitiator allowing the crystallization of the molten corpus to startfrom a single point and the crystallized corpus to be single crystallineand/or have a uniform crystal orientation. Such uniformity would allowthe crystalline islands formed by channels 2220 a-d to have the samecrystal orientation as one another. As some processes used during thefabrication of electronic devices (e.g. oxide growth) can be dependenton the crystal orientation, uniformity between the crystal orientationof the various crystalline islands can allow for greater uniformity inthe fabrication of electronic devices on/in those crystalline islands.

While FIGS. 22a-b depict notch 2240 as the crystallization initiator, itis contemplated that any other suitable feature in any surface ofreservoir 2230 can be used to initiate and/or control crystallization ofthe molten corpus. For example, and without limitation, thecrystallization initiator can comprise a depression into and/orprojection from the surface of reservoir 2230. In some implementations,such depressions and/or projections can comprise a sharp and/or angledfeature to promote and/or initiate crystallization. In someimplementations, the crystallization initiator can comprise anadditional component and/or different material secured to the surface ofreservoir 2230 such that the additional component and/or differentmaterial would come into contact with the molten corpus.

In implementations where the crystallization initiator is a feature in asurface of reservoir 2230, the cooling profile and/or temperatureprofile of the components in contact with the molten corpus (e.g. thetemplate, the substrate, and/or any surrounding atmosphere/gases) can becontrolled to promote the wave of crystallization starting from thereservoir and propagating through the channels, and towards thesubstrate to enable the crystallization initiator to influence crystalgrain(s) and crystal orientation of the crystallized corpus.

While FIGS. 19-22 depict templates comprising one or more channelsformed in the body of the template, it is contemplated that differentsuitable shapes, geometries, and/or structures can be used for thetemplate. For example, and without limitation, the template can compriseone or more channels that are separately formed and then securedtogether. In other implementations, the template can comprise multiplepieces that cooperate to form the channels. In such an implementation,when the crystallized corpus and/or the crystalline island(s) is formed,the various pieces of the template can be separated and/or removed fromone another to liberate the crystallized corpus and/or the crystallineisland(s).

The above-described implementations of the invention are intended to beexamples of the present invention and alterations and modifications maybe effected thereto, by those of skill in the art, without departingfrom the scope of the invention which is defined solely by the claimsappended hereto.

I claim:
 1. A method of fabricating a crystalline island of an islandmaterial, the method comprising: depositing the island material on asubstrate; heating the substrate and the island material, the heatingmelting the island material to form a molten corpus, the heating alsoforming a molten disk comprising oxygen and the island material, themolten disk disposed between the molten corpus and the substrate; andcooling the substrate, the molten corpus, and the molten disk tocrystallize the molten corpus to form a crystallized corpus, at least aportion of the crystallized corpus forming the crystalline island of theisland material.
 2. The method of claim 1, further comprisingplanarizing at least a portion of the crystalline island to expose across-section of the crystalline island.
 3. The method of claim 1,further comprising, after the cooling: over-coating the crystallineisland and the substrate with an over-coating layer to form a stack; andplanarizing the stack to expose a cross-section of the crystallineisland.
 4. The method of claim 1, further comprising, before thedepositing: forming an oxide layer on the substrate; and wherein: thedepositing comprises depositing the island material on the oxide layer;and the molten disk comprises the oxide layer in a molten state.
 5. Themethod of claim 4, wherein the forming the oxide layer comprisesdepositing on the substrate the oxide layer comprising an oxide of theisland material.
 6. The method of claim 5, wherein the depositingcomprises depositing the oxide layer according to a predeterminedpattern.
 7. The method of claim 4, wherein the forming the oxide layercomprises: depositing the island material on the substrate according toa predetermined pattern; and oxidizing the island material.
 8. Themethod of claim 4, wherein the heating comprises heating the substrate,the island material, and the oxide layer in a non-oxidizing atmosphere.9. The method of claim 1, further comprising one or more of, before thedepositing: polishing the substrate according to a predeterminedpattern; and roughening the substrate according to the predeterminedpattern.
 10. The method of claim 1, wherein the substrate comprisesalumina.
 11. The method of claim 10, wherein the molten disk furthercomprises aluminum originating from the substrate.
 12. The method ofclaim 1, wherein the island material comprises silicon and the heatingcomprises heating the substrate and the island material to at leastabout 1500° C.
 13. The method of claim 1, wherein: cooling the moltencorpus forms a first solid portion distal from the substrate and asecond solid portion proximal the substrate, the first solid portionseparating spontaneously from the second solid portion during thecooling, the second solid portion forming the crystalline island. 14.The method of claim 1, wherein the depositing comprises: positioning atemplate on a surface of the substrate, the template comprising achannel having a first end abutting the surface and a second endopposite the first end, the surface capping the first end; and fillingat least a portion of the channel with the island material.
 15. Themethod of claim 14, further comprising: after the cooling, removing thetemplate from the substrate.
 16. The method of claim 15, wherein afterthe removing, a first portion of the crystallized corpus remains on thesubstrate to form the crystalline island and a second portion of thecrystallized corpus remains in the channel.
 17. The method of claim 14,wherein the channel comprises: a first region proximate the first end,in the first region the channel having a first cross-sectional area; anda second region distal from the first end, in the second region thechannel having a second cross-sectional area, the first cross-sectionalarea different from the second cross-sectional area.
 18. The method ofclaim 17, wherein an inner surface of the channel defines a vertexseparating the first region from the second region.
 19. The method ofclaim 14, wherein: the template further comprises one or more furtherchannels, each further channel having a corresponding first end abuttingthe surface and a corresponding second end opposite the correspondingfirst end, the surface capping the corresponding first end; and thedepositing further comprises filling at least a portion of the one ormore further channels with the island material.
 20. The method of claim14, wherein the second end is in communication with a reservoirconfigured to store the island material for at least partially fillingthe channel with the island material.
 21. The method of claim 20,wherein the reservoir is integrally formed with the template.
 22. Themethod of claim 20, wherein the reservoir comprises a crystallizationinitiator, the crystallization initiator comprising one or more of adepression into and an extension from a reservoir surface, thecrystallization initiator configured to come into contact with themolten corpus and initiate crystallization during the cooling.